release: 3.40.0 — file-tree filter

Bump VERSION/pyproject/PWA header to 3.40.0 and the SW cache to
simple-v485 (index.html changed).  CHANGELOG [3.40.0] documents the
file-tree filter (checkbox multiselect, path-exact DF hiding, expand/
collapse all, auto-expand, and the "To EF filter" cross-link).

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v484';
+const CACHE = 'simple-v485';
 const URLS = [
   'index.html',
   'help.html',